Tomographic reconstructions of conventionally-prepared, pastic embedment of isolated rat liver mitochondria were made. Manual outlining in 3-D using Sterecon is still required to segment the membranes in the reconstruction of mitochondria with normal morphology, however, because the contrast of the membranes against the matrix is too low to make use of the segmentation by density or density gradient new specimens have been prepared which enhanced membrane staining using a special en bloc osmium stain. Reconstructions are being made from portions of mitochondria scanned at even higher resolution to study inner/outer membrane relationships.